This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115d. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-10-26

2451. promise.unhandled_exception() and final suspend point

[Accepted as a DR at the November, 2022 meeting.]

According to 9.5.4 [dcl.fct.def.coroutine] paragraph 14,

If the evaluation of the expression promise.unhandled_exception() exits via an exception, the coroutine is considered suspended at the final suspend point.

However, the “final suspend point” is defined as being “the await-expression containing the call to final_suspend” (bullet 5.2), and it is not desired to evaluate the final_suspend expression in this case.

Suggested resolution [SUPERSEDED]:

1. Change 9.5.4 [dcl.fct.def.coroutine] paragraph 5 as follows:

...where

• the await-expression containing the call to initial_suspend is the initial suspend point await expression, and

• the await-expression containing the call to final_suspend is the final suspend point await expression, and

• initial-await-resume-called is initially false and is set to true immediately before the evaluation of the await-resume expression (7.6.2.4 [expr.await]) of the initial suspend point await expression, and

• ...

• promise-constructor-arguments is determined as follows: overload resolution is performed on a promise constructor call created by assembling an argument list with lvalues p₁ ... p_n. If a viable constructor is found (12.2.3 [over.match.viable]), then promise-constructor-arguments is (p₁, ..., p_n), otherwise promise-constructor-arguments is empty. , and

• A coroutine is suspended at a final suspend point if it is suspended

  • at a final await expression or

  • due to an exception exiting from unhandled_exception()

2. Change bullet 3.2 of 7.6.2.4 [expr.await] as follows:

Evaluation of an await-expression involves the following auxiliary types, expressions, and objects:

• ...

• a is the cast-expression if the await-expression was implicitly produced by a yield-expression (7.6.17 [expr.yield]), an initial suspend point await expression, or a final suspend point await expression (9.5.4 [dcl.fct.def.coroutine]). Otherwise

• ...

3. If needed, change 9.5.4 [dcl.fct.def.coroutine] paragraph 14 as follows:

If the evaluation of the expression promise.unhandled_exception() exits via an exception, the coroutine is considered suspended at the final suspend point and the exception propagates to the caller or resumer.

Notes from the August, 2020 teleconference [SUPERSEDED]:

CWG expressed some concern about the lack of a precise definition of “suspend point”. Gor Nishanov suggests the following change, in 7.6.2.4 [expr.await] bullet 5.1:

• If the evaluation of await-suspend exits via an exception, the exception is caught, the coroutine is resumed, and the exception is immediately re-thrown (14.2 [except.throw]). Otherwise, control flow returns to the current coroutine caller or resumer (9.5.4 [dcl.fct.def.coroutine]) without exiting any scopes (8.7 [stmt.jump]). The point in the coroutine immediately prior to control returning to its caller or resumer is a coroutine suspend point.

Proposed resolution (approved by CWG 2022-11-10):

1. Change in 7.6.2.4 [expr.await] bullet 3.2 as follows:

   Evaluation of an await-expression involves the following auxiliary types, expressions, and objects:

   • ...

   • a is the cast-expression if the await-expression was implicitly produced by a yield-expression (7.6.17 [expr.yield]), an initial suspend point await expression, or a final suspend point await expression (9.5.4 [dcl.fct.def.coroutine]). Otherwise

   • ...

2. Change in 7.6.2.4 [expr.await] bullet 5.1 as follows:

   • If the evaluation of await-suspend exits via an exception, the exception is caught, the coroutine is resumed, and the exception is immediately re-thrown (14.2 [except.throw]). Otherwise, control flow returns to the current coroutine caller or resumer (9.5.4 [dcl.fct.def.coroutine]) without exiting any scopes (8.7 [stmt.jump]). The point in the coroutine immediately prior to control returning to its caller or resumer is a coroutine suspend point.

3. Change in 9.5.4 [dcl.fct.def.coroutine] paragraph 5 as follows:

   ...where

   • the await-expression containing the call to initial_suspend is the initial suspend point await expression, and

   • the await-expression containing the call to final_suspend is the final suspend point await expression, and

   • initial-await-resume-called is initially false and is set to true immediately before the evaluation of the await-resume expression (7.6.2.4 [expr.await]) of the initial suspend point await expression, and

   • ...

   • promise-constructor-arguments is determined as follows: overload resolution is performed on a promise constructor call created by assembling an argument list with lvalues p₁ ... p_n. If a viable constructor is found (12.2.3 [over.match.viable]), then promise-constructor-arguments is (p₁, ..., p_n), otherwise promise-constructor-arguments is empty. , and

   • a coroutine is suspended at the initial suspend point if it is suspended at the initial await expression, and

   • a coroutine is suspended at a final suspend point if it is suspended

     • at a final await expression or

     • due to an exception exiting from unhandled_exception()

4. Change 9.5.4 [dcl.fct.def.coroutine] paragraph 14 as follows:

   If the evaluation of the expression promise.unhandled_exception() exits via an exception, the coroutine is considered suspended at the final suspend point and the exception propagates to the caller or resumer.